Allodynia: When Touch Hurts But Shouldn't
Gretchen E. Tietjen, MD
- Allodynia is the experience of pain from a non-painful stimulation of the skin, such as light touch.
- The type of migraine, the frequency and severity of attacks, and the number of years of the headache disorder relate to the presence and severity of allodynia
- Migraine and related pain conditions with allodynia have in common the process of central sensitization, characterized by over excitability of pain nerve cells in the brain and spinal cord
- Triptans are less effective in migraine attacks with allodynia
Allodynia means "other pain." It is a pain that results from a stimulus that is not normally painful. Anyone who has ever had bad sunburn has an idea of how painful even light touch can be. When the skin is sensitized, in this example from the sun, wearing a shirt or taking a shower can be very painful. Up to 80% of persons with migraine experience at least one symptom of allodynia during a headache attack. They may describe pain to touch, such as with resting one’s head on a pillow, or with wearing a hat, earrings, or necklace. These are examples of "static" tactile or mechanical allodynia. An example of "dynamic" allodynia is pain from lightly brushing one’s hair. Another is pain from shaving one’s face. Yet another, thermal allodynia, refers to pain due to exposure to either warm or cold. Allodynia is not referred pain, although it can occur outside the area stimulated. It is also not hyperalgesia, which is a pain stimulus more painful than usual.
The severity of allodynia varies widely from person to person. You rate allodynia by the frequency that each symptom occurs and also by how many different symptoms are present. Headache related allodynia occurs more often in migraine than in tension headaches among headaches without a secondary cause. In persons with migraine, allodynia is more common in those with aura, with frequent headache attacks (chronic migraine), and with severe disabling headache attacks. Allodynia is associated with more disability when objectively measured by MIDAS (Migraine Disability Assessment Scale). The more years a person has migraine the more likely they are to experience allodynia. Women suffer from migraine and allodynia more often than men. Female reproductive hormones lower the intensity of the pain stimulus needed to produce pain. This process known as the pain threshold likely involves processes similar to allodynia. Research shows that being overweight or obese and smoking predispose to allodynia. Obesity is more common in persons with chronic migraine and this may account for the relationship to allodynia. Like female hormones, smoking reduces the pain threshold probably explaining its connection. A stressful childhood marked by emotional abuse increases the frequency of headache and allodynia as an adult. All these facts suggest allodynia is a risk factor for progression to worse headaches. These interesting clinical facts raise questions. Should we use allodynia as a marker for beginning prevention therapy? What causes allodynia? Does the presence of allodynia have an effect on migraine treatment?
Research suggests that allodynia in migraine results from a process within the brain and spinal cord, which make up the central nervous system. Scientists call this process leading to allodynia, "central sensitization". The process begins when sensory pain nerves, known as nociceptors, react to a sensory signal or stimulus. In the case of migraine, inflammation surrounding the blood vessels on the brain surface plays a role. The nerve endings around the blood vessels send signals along the nerves to the central nervous system. This can result in the throbbing head pain of migraine. Acute pain treatment that works stops the signals coming in from the peripheral nerves. When these signals are not stopped the spinal cord and brainstem nerves continue to send their own signals and maintain the pain of migraine. Within as few as one to two hours of activity they become free of what started them in the first place. They become sensitized or hyperexcitable. This is central sensitization. As a consequence, even normal signals into the system, such as touch on the scalp and face, produce abnormal painful responses. The pain feels as if it is coming from the skin but is really the result of a mixed-up processing of sensory signals within the central nervous system. Frequent severe attacks of migraine, particularly migraine with aura, over a long period of time lead to an increased tendency for central sensitization. Allodynia is a manifestation of this central sensitization.
For most persons allodynia resolves when the migraine pain resolves. In some persons allodynia may persist long after the migraine headache subsides. If headaches become daily, allodynia may even become a daily continuous condition. So what, if anything, does allodynia mean in terms of treatment? Treatment effects are unknown for many drugs and treatments. Certain migraine-specific drugs, the triptans, are often less effective when tested in migraine attacks without allodynia. For this reason it is important to treat acute attacks early before allodynia appears. If possible treat before the pain begins to throb. Throbbing is a sign of peripheral sensitization. This process precedes central sensitization. Unlike central sensitization, peripheral sensitization is easy to stop by triptans and other effective migraine therapies. Studies show that some medications work when allodynia is present. Ketorolac, a nonsteroidal anti-inflammatory drug, and dihydroergotamine work when injected with allodynia present.
People with migraine often experience other chronic pain conditions, such as fibromyalgia, irritable bowel syndrome, and chronic fatigue syndrome. Fibromyalgia is characterized by tender painful muscles above and below the waist. Like chronic migraine and chronic tension-type headache, scientists believe central sensitization also causes these chronic conditions. These disorders make up a group known as the central sensitivity syndromes. Migraineurs with allodynia are more likely to have these other pain conditions that those without allodynia, as might be expected. Even when these conditions occur without migraine, they may occur with allodynia. Furthermore, the severity of allodynia in migraine correlates with the number of different pain syndromes, offering further evidence that a common underlying cause may link these conditions together. It may be that if central sensitization develops with one condition, this predisposes a person to developing additional pain conditions. Depression and anxiety are also common in persons with allodynia-associated migraine and the other pain syndromes, but the cause for this link remains uncertain.
Allodynia arises from the brain. Stopping pain signals early can ward off allodynia. Preventing pain from coming again can ward off chronic allodynia. Tested treatments are less effective with allodynia so treat before it occurs. If allodynia occurs frequently use prevention to reduce risk of it occurring often.
Gretchen E. Tietjen, MD, Professor and Chairman of Neurology, University of Toledo, Toledo OH.